This invention relates to traction drive torque transmissions and, more particularly, it concerns improvements in traction surface cooling apparatus for such transmissions.
In traction drive transmissions, torque is transmitted by rolling friction between one or more pairs of traction surfaces on components arranged to be retained against one another in a manner to develop normal forces adequate to prevent slippage between the surfaces. Such transmissions are particularly useful in the transmission of power at continuously or infinitely variable speed ratios because of the facility offered by the smooth rolling surfaces of each traction surface pair for an infinitely variable radius ratio. Examples of such infinitely variable transmissions are disclosed in U.S. Pat. Nos. Re. 29,328, reissued Aug. 2, 1977, No. 4,112,779 and 4,112,780, both issued Sept. 12, 1978, and U.S. Application Ser. No. 706,291, filed July 19, 1976, now Pat. No. 4,152,946 all of which are owned by the assignee of the present invention.
Though seemingly inconsistent with transmission of torque by friction, the rolling or traction surfaces of the transmission exemplified by the disclosures of the aforementioned patents and application are lubricated and cooled by circulating a liquid lubricant through the transmission housing. Torque transfer is, in actuality, by viscous shear of a very thin film of lubricant between the traction surfaces which are of smooth tool steel. The lubricants used are synthetic oils developed specifically for traction drives and increase in viscosity under the pressures existing between the traction surfaces to a point of becoming almost glassy in character. Accordingly, high coefficients of traction are possible without abnormal deterioration of the contacting surfaces.
As indicated, the liquid lubricant functions also as a heat storage medium by which the heat developed at the traction surfaces is transferred to the exterior of the transmission housing by recirculation and cooling of the lubricant. Partially because of the relative motion between the traction surfaces and the recirculated lubricant, and also in part because of the viscosities reached by the lubricant, a boundary layer of lubricant tends to build on the surfaces to a point where the torque transmitting efficiencies of the lubricant is reduced and more critically, the transfer of heat to the recirculated lubricant is impeded. These problems created by the boundary layer of lubricant are, moreover, dichotomous in the sense that the reduction of torque transmitting efficiency can be avoided by circulating less lubricant over the surfaces whereas the removal of heat developed by the stresses imposed on the traction surfaces requires large quantities of the lubricant to be circulated over the same surfaces. Hence, the solution of these problems in the past have involved a trade-off or compromise between rated power transmission capacity and useful life of a particular transmission unit.